1. Field of the Invention
This invention relates to a method defined in the introductory part of claim 1 for managing a buffer in a data network, in which the data is divided into frames and the frame has several cells.
2. Description of the Related Art
At the present high-speed data networks are used for the data transfer, of which an ATM-network (Asynchronous Transfer Mode)-network can be mentioned as an example. Besides high transfer speeds the most significant advantages of the ATM include a connectable and dynamic architecture. ATM can easily be adapted for different occasional telecommunication speeds. ATM- terminal devices can also reserve the desired portion of the band width of the ATM-network in order to establish connections similar to a telephone network. Due to the connection distance one of the most important factors influencing the ATM performance is the architecture of the coupling circuit in the ATM-device.
When information is transferred between two computers through private and public ATM-networks, the transmitter must negotiate with each node or node point along the way concerning the routing of the packages, transfer speeds and service levels. The process is in principle analoguous to the connection establishment in a telephone network. The most remarkable difference is the determination of the service level. The communications description defines the parameters for the average transfer speed of ATM-cells as well as the burst duration and the delay variation tolerance. It is also determined, whether the connection serves constant bit rate or variable bit rate sources and whether the question is about connectable, as frame relay, or nonconnectable communications known from the local network.
Depending on the agreed service level the nodes follow priorities, which determine, which of the willing cells is allowed firstly inside the ATM-circuit. Besides it is determined by priorities, which of the cells are removed firstly, when the transfer channel is for some reason overloaded.
An ATM-connection is established by paths (Virtual Path) formed inside a physical copper or light cable and connecting together nodes and terminal devices, which paths for their part are divided into several logical virtual channels. By paths the stationary reserved connections are realized and by channels the connectable temporary connections. Thanks to the flexible architecture each parallel channel of one path can transfer data of different type. The conception of virtuality peculiar to ATM is therefore generated by the idea, that the physical movement of a cell through the ATM coupling circuit lasts only for a while, though the connection is available all the time.
The fixed length of an ATM-cell has been chosen as 53 bytes, of which 48 bytes include actual information. In order to save space the 5-byte header of the ATM-cell does not comprise all necessary routing information, but only data of the next path (VP) and channel (VC). ATM assumes that the underlying transfer path is reliable, whereby the node points do not check the integrity of the data they transfer.
The path usually comprises several channels seeking for the transfer capacity. It is therefore obvious that in a certain stage several packages are simultaneous striving to attain the node. On the other hand the dynamically varying transfer speed may cause in the case of burst-filled communications situations, in which the optimal transfer speed of the node point is exceeded. Inner blocking takes place in the circuit itself for example in the case, when two data packages going to different out-ports are obliged to use the same inner route. If the circuit is not able to adapt two cells simultaneously to the same route, the other cell must be buffered or even destroyed. Upon the delivery blocking two cells are competing for the same out-port. By placing the buffer to the out-port, part of the problems is avoided. The buffer may operate either by the FIFO- line principle or according to priorities, whereby the time-depending audio and video files can be transferred ahead the other data.
When fixed transfer speeds are used in a node point, jamming does cause problems. Troubles emerge only when the communication quantity can sway together with random data buffers. Inner buffering of the node point eliminates short peak loads. The buffer seizes all the data packages, whose speed exceeds the handling ability of the node. Transfer of packages to the circuit takes subsequently place either by queueing or priority. For the jamming control also different priorite levels are needed for normal data and control information.
The structure and control of buffers may be the most important of the details connected to the ATM-coupling or node point. Buffering is needed in order that switches would not lose packages or frames in burst-filled traffic. Buffers are on the other hand harmful, because their construction is costly and as large devices they cause irritating delays. The buffer size depends on the circuit architecture and the adaptation. Most nodes have been planned on the starting buffers, whereby the packages go through the node before stopping. Also inner input and node buffers have been used.
The deeper into the buffer the data must be transferred, the more harmful delay is generated. Therefore the port-related buffers should be as short as possible. The node should mostly comprise space for at the most 50-256 cells to be buffered per port. In greater package amounts one must rely upon the traffic balancing techniques of the ATM-network.
A substantial part of the ATM-network traffic comprises data communications, especially between local networks. Typical for this kind of traffic are very great variations, the anticipation of which is difficult. On the other hand the data communications can often be adapted to the variations of the available capacity. For the most effective utilization of the ATM-network capacity the network can announce to the user the highest permissible transfer speed of the connection. For this a so-called ABR (Available Bit Rate) -service is under development. Another possibility is that the network does not inform the overloading situations to the users, but the user observes them as loss of data frames (Unspecified Bit Rate, UBR-service). In both cases it is very important that losses of frames are distributed fairly to different connections. Fair buffering causes, however, a significant problem, because transfer speeds in the ATM-network are very high, whereat realization of complicated protocolls and buffering methods causes great additional costs. Buffers with large physical size are also often used, which in this case require valuable space in the circuit board and engender extra costs.
Some methods for buffer control are already known. In the weighted Fair Queueing (WFQ) -method certain portion of the buffer capacity of each node is reserved for each connection. The number of buffer places needed by each connection depends on the traffic parameters defined for the connection. By this principle it is possible to guarantee for each connection (following the agreed traffic parameters) a trouble-free service regardless other traffic in the network.
By WFQ it is possible to guarantee trouble-free service simultaneously for very different connections. Its technical realization is however difficult, because it requires both a sophisticated buffering system and a complicated method, by which it is determined, which cells are accepted to the buffer. Static distribution of the buffer capacity based on WFQ requires very large buffers in each network node, if the number of connections is high.
Another known solution is a so-called Random Early Drop (RED), in which the data frame to be dropped during the overloading situation is randomly selected. The RED-principle is suitable for networks without connections, in which there is no information about the connections and their amounts. The ATM-network is, however, connectable in nature, so that rejection of frames may be done on the basis of data related to the connections.
A third solution is based on the fact that the network informs the users about the overloading situation in the network using e.g. a method of the EFCI-type (Explicit Forward Congestion Indication). When the network has sent an EFCI-notice, it is assumed that the users reduce the speed of the traffic to be sent to the network.
The solutions of both the RED and EFCI-type are simple from the view of the buffering realization. By them it is, however, very difficult to protect other network users, if some connection does not follow in advance agreed procedures in the overloading situation. By these methods it is not therefore possible to realize a fair capacity distribution. In large networks the influence of an EXCI-notice begins after a delay of considerable length, even tens of milliseconds. In order to keep moderate the amount of lost cells and frames, very large buffers are needed in each network node. EFCI requires equipment alterations in all parts of the network from connection cards to network nodes. Further on the most available embodiments cannot react to an EFCI-notice.